I. Field of the Invention
This invention relates generally to electromedical circuit apparatus and more specifically to an electronic interface module which is adapted to be coupled between conventional body contacting electrodes and patient monitoring equipment, the interface module acting to suppress or eliminate any aberration which may be superimposed upon the physiological signal being detected by externally applied body tissue stimulating impulses.
II. Discussion of the Prior Art
It is now quite commonly known that a transcutaneous electronic nerve stimulator (TENS) device may be used following surgery for the treatment and suppression of pain. In a typical TENS device, electrical impulses of predetermined frequency, amplitude and pulse width are generated and applied to the body of a patient proximate the locus of the pain by way of body contacting electrodes.
In modern post-operative intensive care units various vital signs are continuously monitored by automatic equipment in such a way that trained personnel at a central station are able to obtain an immediate indication of variations in such vital signs for a large number of patients. For example, following thoracic surgery, the patient is wired with suitable body contacting electrodes such that his EKG trace can be monitored at a nurse's station. Heretofore it has not been possible to treat a patient whose EKG trace is being monitored with a TENS device because the regularly applied stimulating pulses used for suppressing post-operative pain would feed through the body tissue to the patient contacting electrodes and would be picked up by the monitoring equipment. The strong stimulating signal would normally overload the sensitive amplifiers used in the monitoring equipment which could result in severe distortion of the waveforms being monitored to the extent that they become useless as a diagnostic tool. For example, the physiological signal comprising the EKG trace is routinely less than 0.001 volt. The TENS signal applied to the patient's skin is typically greater than this signal by a factor of up to 200,000.
Further, typically the body stimulating pulses from the TENS unit are of a substantially higher repetition rate than the PQRST complex of the typical EKG trace. As such, if one were to attempt to monitor the EKG trace while the patient is undergoing treatment with a TENS unit, one would find very high amplitude spikes superimposed upon the EKG trace. Depending upon the type of patient monitor being used, these spikes may appear as a fogging of the EKG signal and in displays employing cathode ray tubes of longer persistence, the presence of the stimulating impulses superimposed upon the EKG trace may cause complete obliteration of the cardiac signal.
Thus, if one is to effectively treat the patient with electrical stimulating pulses at the same time that it is desired to continuously monitor the patient's physiological processes, it becomes necessary to devise a way in which the body stimulating impulses can be discriminated and blocked so that they do not reach the display portion of the monitoring equipment.
In typical electrocardiographic equipment, highly sensitive direct coupled amplifiers are commonly employed to magnify the minute electrical signals comprising time varying voltages, which are generated by the body so that these signals may be recorded and/or observed. A difficulty in using such high sensitivity instruments is that in addition to the desired signal, there may be extraneous interfering signals and noise superimposed upon the desired signal which tend to obscure, distort or even overshadow the desired signal by reason of their magnitude, to such an extent that the value of the record of the desired signal is substantially decreased. In attempts to correct this situation, prior art approaches have relied upon frequency discrimination (filtering) or signal cancellation in which the interference signal is amplified and inverted in phase so that the resulting mirror signal can be used to cancel out the original interference signal.
These prior art techniques for cancelling noise components of a desired signal which are found in present day patient monitoring equipment are found to be ineffective in dealing with the intentionally applied body stimulating impulses used in the treatment of post-operative pain. Because of the direct application of the stimulating impulses to the body of the patient by way of skin contacting electrodes, the magnitude of the pulses picked up by the monitoring electrodes is so large in comparison to the physiological signal being measured thereby that the electronic equipment designed to respond only to the physiuological signals tends to be swamped out.
The purpose of the present invention is to provide an electronic interface module that will permit patient monitoring to take place simultaneously with the treatment of the patient involving the application of relatively high frequency stimulating impulses to the patient such as with a TENS device. Through the use of the module comprising the present invention it is possible to recover the relatively low level physiological signal even when buried in the much higher amplitude, much more rapid stimulator pulses.